The optical measurement of light spot position is used in many robotic control and vision systems. Range cameras, used for three dimensional (3D) imaging, exploit geometric arrangements (such as active optical triangulation), properties of light propagation, and light interference to extract distance shape. Optical triangulation relies on the measurement of the position of a focused light beam impinging on a linear or bi-dimensional sensor.
FIG. 1 illustrates a basic light position measurement system 10 according to the prior art. The system includes a lens 12 for receiving light beams 14a,b generated from a projected beam 16. An optical position sensor 18 receives the light beams 14a,b after passing through the lens 12 emanating from the intersection of the projected beam 16 with a reflective surface at 19a and 19b.
Optical triangulation is based on the sine law where the knowledge of two angles of a triangle and one side fully determines its dimensions. As shown in FIG. 1, two known angles .varies..sub.1, and .varies..sub.2 of a triangle 20 (bounded by the projected beam 16, the collected beam 14a and a baseline 22) are the angles of the projected beam 16 and the collected beam 14a relative to the baseline 22, respectively. The angle .varies..sub.2 of the collected beam 14a is measured using the lens 12 and the optical position sensor 18 that measures a length L as the distance between the collected means 14a,b on the sensor 18. The length L is related to the angle .varies..sub.2 and therefore a displacement Z based on the sine law as discussed above.
Many devices have been proposed in the prior art for measuring the position of the collected light beams. The devices generally belong to one of two main groups: continuous response position sensitive detectors (CRPSD) and discrete response position sensitive detectors (DRPSD).
A CRPSD is defined as a class of position sensitive detectors that determinelcalculate the centroid of a light distribution, which may include stray light components in addition to a desired light spot. A DRPSD is defined as a class of position sensitive detectors that samples and analyses the entire light distribution to determine the position of the desired light spot within the light distribution.
CRPSD are generally based on lateral effect photodiodes and geometrically shaped photo-diodes (wedges or segmented) such as disclosed in A. Makynen and J. Kostamovaara, Linear and sensitive CMOS Dosition sensitive photodetector, Electronics Letters, Vol. 34 (12), pp. 1255-1256, Jun. 1998 and in A. Makynen et al., High accuracy CMOS position sensitive Photodetector (PSD), Electronics Letters, Vol. 33 (21), pp.128-130, Jan. 1997.
DRPSD are generally implemented using an array of photosensors where they are read out serially by metal oxide semiconductor field effect transistor (MOSFET) switches or a charge coupled device (CCD) such as disclosed in F. Blais and M. Rioux, Real-Time Numerical Peak Detector, Signal Process., 11(2), 145-155 (1986).
CRPSD are known to measure the centroid of a light distribution impinging on its resistive surface to a very high level of resolution and speed. However, accuracy is reduced when spurious light is present together with the main light distribution. DRPSD can achieve, with a peak detection algorithm, higher accuracy levels since the distribution is sampled and hence available for processing but at a slower speed relative to a CRPSD.
Consequently, there is a need for a position sensitive detector that provides high resolution and speed, as offered by traditional CRPSD, together with high accuracy under variable light conditions, as offered by traditional DRPSD.